Thin metal strip a few mm in thickness may be cast directly from liquid metal (steel or copper, for example) on a plant referred to as a “twin-roll casting” plant fed with liquid metal from a tundish by means of a pouring nozzle. The machine includes a mould, the casting space of which is defined along its long sides by a pair of internally cooled cylinders with parallel horizontal axes and rotating about these axes in opposite directions, and along its short sides by closure plates (called side walls) made of refractory, these being applied against the plane ends of the rolls. The liquid metal must solidify only on the cooled cylindrical surfaces of the rolls, by forming solidified shells that join up in the nip (the region where the distance between the surfaces of the rolls is a minimum) in order to form the strip, which is continuously extracted from the casting space.
In practice, it is difficult, however, always to avoid the appearance of solidification referred to as “spurious solidification”, that is to say solidified metal layers that are created in certain regions of the side walls. They are due to the fact that the liquid metal neighbouring these regions may be at a temperature substantially below its nominal temperature and therefore liable to solidify locally on contact with the side walls. This low temperature may be due to unfavourable hydrodynamic conditions which do not allow the liquid metal in these regions to be sufficiently replenished, or at an insufficient temperature of the side walls due to poor preheating before casting. Of course, several of these factors may combine. When the solid metal resulting from this spurious solidification is entrained into the bottom of the casting space, it must pass between the rolls, creating a thickness of metal which is added to the normal thickness of the product. It follows that the rolls must momentarily absorb an additional load which forces them to temporarily move apart, in order to avoid damaging them. The quality of the strip is unfavourably affected thereby. It is also possible to have the side wall pushed away, with the risks of a loss of sealing of the casting space that are associated therewith.
Usually, it is attempted to limit the appearance of spurious solidification by adopting particular pouring nozzle configurations. The aim of these is to impose on the liquid metal in the casting space flow conditions that are assumed to ensure continuous replenishment of the metal facing the regions where the spurious solidification is most likely to occur, for example by bringing liquid metal leaving the nozzle directly into proximity with these regions. However, this may result in a lack of liquid metal feed in the other parts of the casting space.
Another method consists in always heating the side walls during casting, by means of burners or induction furnaces, or even also induction heating the metal lying near them. However, this results in complications in the construction of the side wall and in managing its operation.
Another method consists in varying the composition of the refractories of the side wall by placing, so as to face the casting space, fibrous refractories based on silica, alumina or other oxides having highly insulating properties. These highly insulating refractories can extract from the liquid metal only a relatively small amount of heat and thus limit the risk of spurious solidification. However, they have a low hardness and therefore withstand poorly the friction against the rolls, and also against the solidified metal or metal undergoing solidification in the vicinity of the nip. This is why, in the regions of the side wall that flank their arcs of contact with the edges of the rolls and in the regions that face the lower part of the casting space, the aforementioned insulating refractory of the inserts is substituted with a material having less insulating power but higher hardness, namely various ceramics, boron nitride, SiAlON®, etc.
However, this solution is not entirely satisfactory as heat transfer between the hard refractory and the insulating refractory occurs in their contact region, resulting in localized cooling of the insulating refractory. This may be sufficient to initiate spurious solidification.
The object of the invention is to provide a side wall designed for the twin-roll casting of thin strip that limits the appearance of spurious solidification better than the existing designs.